With the Wrong Person
by RedDestiny123
Summary: Derek discovers something about Casey that he really wish he hadn't. *RATED M FOR FUTURE CHAPTERS*
1. Chapter 1

**AN: I'M BACK! I hope you like this. The idea came to me when I was half-asleep, haha. But I hope I did well... Enjoy!**

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 **With the Wrong Person**

 **Chapter One**

"DER-EK!"

Derek smirked because he always knew what that sound meant. He was in trouble. And this time he had a feeling he knew why Casey was yelling. So he walked into her bedroom with his signature smirk. "You rang, sis?"

" _Step_ -'sis,'" she corrected him, putting quotations marks around 'sis.' "Now, what the hell is this? she questioned, holding up her diary, which now had a broken lock.

Derek's smirk hadn't faltered throughout Casey's little 'freak out.' "Well, I'm no genius, but it looks like a diary," he answered snarkily.

Casey narrowed her eyes, giving him a sneer. "No shit, Sherlock. But I know you did this!" she announced, pointing to the broken lock.

Derek just shrugged nonchalantly. "Yeah... What of it?" he questioned. "You should know by now that, if something is yours and it's meant to be a secret, I'll find out what it is," he reminded her with a raised brow. "But, for once, I wish I hadn't invaded your privacy."

Casey matched his raised brow, only in confusion. That is, until she realized what was written in the small book. "DER-EK!" she screamed out once again.

Derek just shook his head in disbelief. "Truman?" he asked for confirmation. "You lost it to Truman?" he continued on.

Casey growled, almost as if she were an angry animal, while lunging herself towards Derek. Though all he had to do was step out of the way since he had a feeling that that was coming.

"Just because no one's home is no reason to get violent," he berated, wagging his finger in mocking disapproval.

"Who I lost my virginity to is none of your business!"

"Listen, Space-Case, it _is_ my business. Contrary to popular belief, there is a part of me, no matter how small that part may be, that cares about you. You gave yourself up to a womanizer."

"Oh, like you're much better?" she shot back.

"Touché," he responded with a sigh. "But I only sleep with girls who have done it before. I don't 'get' with virgins unless I really love them. Truman doesn't do love... And why the fuck am I explaining myself to you?" he inquired, more to himself than her.

"I don't know and I don't care. All I know is that you butt into my personal life unwarranted. Now get out of my room," she demanded in a very un-Casey-like manner.

Derek put his hands up in fake defense with a slight roll of his eyes, an irritated sigh to go along with it. "Just prepare yourself to find out that you have herpes," he playfully warned. Though apparently Casey didn't find his joke as funny as he did. It earned him a slap to the face and being physically pushed out of her room.

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 **AN: I hope it was good. I know it's been a while since I've updated... So I'm still getting into the groove, haha. Now, don't forget to review and tell me what you think!**


	2. Chapter 2

**AN: I appreciate the reviews from the last chapter. Here's the next!**

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 **Chapter 2**

There was a loud knock on the front door and Derek had a strong feeling that he knew who it was. So, before Casey could get downstairs. the brunet got off of his recliner to open the door. Ah, he was right. It was, indeed, Truman. "Hey, man! Long time, no see!" he spoke with the fakest smile that he'd made throughout his whole life. Truman was noticeably uncomfortable.

"Uh... Yeah... The last time I saw you was at the party..."

Derek pretended to ponder this fact before giving a devilish grin. "Oh! Right! The one where you cheated on Casey with her own cousin! Good times..." Though, before he could continue his 'conversation' with Truman, the young male heard the pounding of feet bounding down the stairs.

"DER-EK!" Casey screamed out. She was really getting tired of her step-brother's antics. "I'm so sorry, Truman. Just wait outside and I'll meet you in the car," she assured, only receiving a timid nod in response as he left and shut the door behind him. Then Casey immediately went after Derek. "What is your problem?!" she demanded of him.

Derek sighed and rubbed his temples. "He looks at you like you're a piece of meat. He takes your heart and does whatever he wants with it. How many times have you come home crying because of him?" he asked, his voice soft though with a hint of dominance hidden in it, as well.

"When the hell did you start caring so much?" she inquired the question that she had wanted to ask since the night before.

Derek stood there, for once, speechless. He thought that he'd explained himself pretty well the night before. A part of him cared about her. That's all there was to it, wasn't it? Great. His grade-grubbing step-sister had him questioning himself. Derek Venturi never questioned himself. But he hadn't realized that he'd been staring at Casey the entire time that he'd been thinking. Not until her voice snapped him out of his trance.

"Der?" She sounded almost concerned.

Then he did the unexpected. Something he never thought he'd do. Derek Venturi kissed Casey McDonald.


	3. Chapter 3

**Chapter 3**

Casey stood still, not exactly sure what to do. Was she supposed to kiss him back? A part of her desired to. But the rational part of her felt as though it was wrong. It had to be, didn't it? They were meant to be family. Though it's not as if they grew up together. They met when they were teenagers... Their parents had to see this coming. Then again, apparently they already knew that their kids would hate each other. So maybe they wouldn't see this coming... After all, the two teens did hate each other practically on sight. Maybe the two adults were relieved. Then she realized that overthinking it would be pointless as she found herself already returning the kiss. Hell, the young brunette had even begun to deepen the lip-lock. It was all instinct at this point.

Derek's eyes had closed, though he did raise each of his eyebrows in sudden surprise. Not just because she kissed him back, but he was also taken aback by just how good of a kisser the keener was. He did his best to ignore how she'd come to acquire that experience. He knew how. It was because of the obnoxious kid outside that was still waiting for her. The young man didn't care that he was keeping his step-sister from going out. Or at least somewhat stalling her leaving. Karma was a bitch and Truman was going to learn that the hard way.

Casey pulled away for air, not having noticed that her hands had travelled up in the heat of the moment into Derek's hair and knotted in it. But she found herself not pulling away and cherishing the feeling.

"Like what you see, princess?" he teased with a smirk.

"Der-ek," she responded playfully, a smile generating on and gracing her lips. Though that precious, meaningful smile immediately fell when she glanced over in the direction of the front door to see Truman standing there.

"I just came to see what was taking so long," he explained, his expression almost seeming as if he were traumatized.

Derek rolled his eyes at Truman's words. "Well, I suppose this is the appropriate, not to mention the perfect, time to tell you to fuck off.

Casey looked to Derek and let out a defeated sigh, finally managing to see what everyone had been telling and warning her about Truman. "I think you should leave," she whispered, her voice wavering, though dominance apparent in her tone.

Truman looked over and between the two step-siblings. "If you wanna leave me for your step-brother, do it. You're both sick fucks."

Derek just smirked. "Sore loser. You were only looking forward to more sex. And what better sex than with a virgin? Or, at least, a past virgin..." he trailed off, making sure that Truman knew that he knew Casey was no longer sexually pure. "You're just disappointed that she left you with some blue balls," he continued to shoot back, leading the other male to the door as he spoke. Once the issue had been removed from the house, the brunet turned around. "Problem solved."

"Not quite... We still have to tell the family," the young woman reminded him.

Derek rolled his eyes once again. "We'll worry about that when they get home."


	4. Chapter 4

**AN: My computer has been out! I'm sorry! But I'm back!**

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 **Chapter 4**

Derek watched Casey pace around the living room, rolling his eyes at how anxious she was acting. "Calm down, Space-Case."

"You're real great at reassuring people," she spoke up, stopping her pacing so she could face him with her arms crossed across her chest.

Derek chuckled and took both of Casey's hands in his own. "You know that they won't be home for another hour or two," he reminded her. "I wanna show you something."

Casey looked at him skeptically before deciding to trust and follow the brunet upstairs to his... bedroom? "What are we doing in here?" she questioned, though she had a feeling that she already knew and didn't need to ask.

"You need to relax," was all he answered with before tugging off his shirt.

Casey's eyes widened. She'd never noticed how muscular Derek actually was. But not disgustingly so. He was the perfect type of fit. Obviously from hockey.

"I can see you can't take your eyes off of me... So do I have to undress you or can you do that yourself?" He was answered with her own shirt being removed. Oh... That dancing definitely _was_ good for something...

Casey wore a smirk that was quite identical to Derek's. "Well?"

"You're... beautiful," he finally spoke up before practically pouncing on her and laying her down on his bed, attacking her neck with kisses and earning multiple moans - especially when he began to suck on her skin. The young male felt her begin to undo his jeans. Almost like an expert. But he pushed that thought out of his head and focused on the now.

Casey tugged down his jeans and boxers simultaneously before beginning to work off her yoga pants. Though she was stopped by Derek's large hands taking her small ones to give himself the opportunity to undress her. The brunette watched with hungry eyes as he pulled down her pants before his lips were on hers. Her fingers found their way into his already-unruly brown locks. Her tongue slithered into his mouth, dancing with his as if they did this every night. Though a part of her had a feeling that this _would_ be happening every night if things continued going at this rate.

Derek pulled away from the lip-lock reluctantly; though there was still work to be done. There were two pieces of clothing that needed to be taken care of. He reached around to undo her bra and toss it to the side, then began to kiss down her chest, sucking on and nipping at each of her nipples while receiving approving moans, before kissing lower to tug her panties down with his teeth. It was quite obvious that he'd done this before; many times, indeed. But that wasn't important at the moment. And it was common knowledge, anyway. The brunet was about to stick his tongue into Casey's cunt, though was surprisingly stopped.

"I want you... _all_ of you... now," she practically demanded, though in a soft tone. But it didn't take much thought as he immediately obliged to her request.

Derek reached into his dresser, fumbling around and underneath his clothes to find his hidden box of condoms, relieved that Casey didn't comment on the fact that he had condoms at the ready. He ripped open the package with his teeth and slid the protection onto his hard cock. The head of his dick rubbed against her opening as he glanced down at her. "Ready, baby?" he whispered, receiving a silent nod in response. Full silence filled the room... Until he pushed into her.

Casey moaned loudly, unable to help herself, as she wrapped her legs around Derek's waist.

"You're so fucking tight," he grunted, pounding into her once the two had adjusted to one another.

The headboard hitting the wall didn't bother either of them. Hell, they could barely hear it above their own moans.

Another thing they didn't hear was the front door opening.

"C'mon, Marti. Let's get you up to your room so you can lay down," George spoke up. They'd expected to be out much longer. But, unfortunately, Marti had gotten sick at dinner. The only blond of the home went upstairs with the little girl in his arms, though stopped in his tracks when he heard the banging and the moans to go along with it. He quickly ran downstairs with a sleeping Marti in his arms (thank God) and handed her to Nora.

"Honey, what is it?" the woman questioned worriedly, though didn't receive a response.

George was already running back upstairs.

"Der-ek!" Casey shouted out in pleasure right before the door burst open.

"What the fuck?!" George demanded, Lizzie and Edwin peeking behind him.

Lizzie smirked and looked over to Edwin who seemed defeated. "Ten bucks," she told him plainly, holding out her hand.

Kids know everything.


	5. Chapter 5

**AN: I'm so sorry that it took so long to update. My Internet is down again… What a surprise, right? I'm actually on my dad's computer right now while we work on my computer. Technology, am I right?**

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 **Chapter 5  
**

Derek gave both Lizzie and Edwin a glare that could put Lucifer to shame. Really? They'd bet on them getting together? "Seriously?" he questioned their younger siblings, almost completely ignoring his father's yelling.

Casey had herself immediately covered up, not about to stand and get dressed to allow everyone to see her nude, so the blanket would have to be her clothing at the moment. "George, I am so sorry… I don't know how this happened… We were fighting and then…" she trailed off, not about to go into detail of their sexual encounter. Not only not with George, but not with anyone. Especially not Emily. She loved that girl to death, but she knew that she also had a big mouth.

George didn't know what to think. Should he have seen this coming? They were the same age… But they hated each other! This had to be some twisted dream. That's what it was.

Nora had put Marti to bed, the young girl being out like a light, before walking over to where the rest of the family was. What was going on? Why was Edwin handing Lizzie ten dollars? She was definitely missing something. Though, once she made her way past her daughter, step-son, and her husband, the woman then realized exactly what was going on. She had to admit that this wasn't much of a surprise. And it's not as though they'd had sex while the family was home. Usually George was the one that was the laid-back parent, but this time _she_ had to be the one to calm things down. "Okay, let's allow them to dress themselves and we'll go from there," she announced, shooing Edwin and Lizzie to their bedrooms. Though, when doing so, she gave them a silent look that spoke a million words. No spying.

Both younger siblings knew that look and were gladly going to oblige. They'd rather not be ripped to shreds. Even if it was just with words. So they disappeared into their rooms and had no intentions of leaving said rooms.

Nora tugged George by the arm to leave Derek's room, no matter how much the blond objected. "Do you really want to talk to them while they're both still naked and in bed?"

Okay, she had a point. So the man obliged reluctantly and the married couple made their way downstairs, sitting at the dining room table.

Casey knew that her cheeks had to be a deep crimson. Her lips were sucked in, for once at a loss for words. All she could think to do was stand and begin getting dressed. The brunette could practically feel Derek's eyes boring into her naked frame. "Take a picture, it'll last longer," she spoke up, her sass returning.

"There's the Case that I know," he commented. None of this bothered him. What were they gonna do? Yell at them? Disown them? Hell, if they disowned them, more sex was more than inevitable. At least, he hoped so. Though he supposed that he should begin getting dressed, too. And, as he did, every worst-case scenario that he could think up only seemed to work in their favor.

Once she was dressed, Casey looked over to Derek and growled when she saw that he was only half-dressed. "Will you hurry up?! They're waiting for us!" she exclaimed.

Derek raised his hands up in mock defense. "Calm it, Space-Case. I didn't realize that you'd be getting dressed at the speed of light," he mentioned with an exasperated sigh, finally getting completely dressed. A part of him wanted to take her hand as they walked down the stairs, but the sane part of him knew that Casey would only see the affectionate gesture as a way to romance her into another fuck. Even if that wasn't the case.

So the two of them just walked down to the dining room silently, sitting across from their parents.

 _God, this feels like a goddamn intervention,_ Derek thought to himself. Maybe it was. An intervention on not having their children fucking. Oh, come on, they didn't grow up together. They weren't related. Neither parents legally adopted the other's children. There was nothing wrong with this. It was legal and he knew that they were both far from being crazy or gross. Casey was hot. He thought that the first time he saw her, even if he was pretending to be Ralph. And, another thing he'd found in Casey's diary when he'd invaded her privacy (as always), was, in the very beginning… when their parents were first getting together… she'd made it very apparent that 'Derek's' friend 'Ralph' was quite attractive, even if he'd been a complete jerk to her when they'd first met. Ah, good times. But enough reminiscing. This wasn't the time for that.

Casey stared at both of the adults, her eyes filled with her natural innocence. Along with guilt… Even if she wasn't guilty about sleeping with Derek. It was more about getting caught. She'd thought that they'd had more time! She didn't mean to scar her family for life! Except Edwin and Lizzie, of course. So immature. Making bets on behalf of their siblings. How low could you get? Then again, they were still kids.

Nora leaned her elbows onto the table and rested her knuckles beneath her chin to look at them. "Okay, you can both stop looking like you've been caught robbing a bank," she began, "because I'd prepared myself for this… even if George hadn't.

A grunt left the back of the older man's throat, his arms crossed over his chest. Great, now he was acting like a child that hadn't gotten his way. He still didn't understand how _Nora_ was seemingly okay with this while _he_ was the one that was planning the punishments in the back of his head.

Casey's gaze shifted to George when she heard him make some sort of noise that obviously was not of approval and certainly not happiness. "I'm so sorry. I don't think I can say that enough," she insisted, feeling Derek's palm on her thigh. "Get away, fly!" she exclaimed, shooing his hand away, causing her step-brother and… lover? to shrug nonchalantly.

Nora did her best not to laugh, so only a snort came out, resulting in her covering her mouth and nose in an attempt to silence her noise of amusement. "Just wait a little longer to finish our conversation. Then you two can go do whatever you feel is necessary."

"Nora!" the blond practically screamed.

"They look at each other like we look at each other… You can't just stop them… They'll find a way," the woman told her husband. Receiving another grunt in response, the mother and step-mother of the teens across from her rolled her eyes at how immature he was acting. "Okay… So… House rules…"

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 **I hope that was enough to make up for the time I wasted and to tide you over for the next chapter! Review and all that good stuff! See you hopefully soon!**


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